FIG. 5(a) shows a cross-section of a conventional solid state imaging element having a light detector (hereinafter referred to as a photodiode) and a charge storage section in which charge skimming takes place.
In FIG. 5, reference numeral 8 designates a P type semiconductor substrate. An N type region 1 having a dopant impurity concentration of N.sub.1 for producing a photodiode 10 is disposed in the P type semiconductor substrate 8. A surface P.sup.+ layer 2 is disposed on the N type impurity diffusion layer 1 so that photodiode 10 is buried to reduce the dark current. An N type layer 3 having a dopant impurity concentration of N.sub.2 which produces a buried type CCD (Charge Coupled Device) is disposed on the surface of the semiconductor substrate 8. A barrier gate 4 is provided for applying a bias to the photodiode 10. A storage gate 5 is provided for producing a storage section 20. A reading out electrode (shift gate) 6 is provided for transferring charges from the storage section 20 to the CCD 30. Reference numeral 7 designates a transfer electrode of the CCD 30.
FIG. 5(b) shows a potential diagram for explaining the operation of the device of FIG. 5(a).
The device will operate as follows.
When light is incident on the photodiode section 10, signal charges are generated. Because electrons flow toward a lower potential, when a voltage is applied to the barrier gate 4 and a voltage smaller than the barrier gate voltage is applied to the storage gate 5, signal charges generated at the photodiode section 10 are transferred to and stored at the storage section 20 through the barrier gate 4. In FIG. 5(b), reference character Q.sub.sig designates stored signal charges. After a predetermined storage time has passed, a voltage higher than the storage gate voltage is applied to the shift gate 6 thereby to read out the signal charges Q.sub.sig to the CCD 30 through the shift gate 6. These signal charges are successively transferred to be output.
In an image sensor which operates in such storage mode, the dark current component is superimposed on the signal charges as noise, and variations in the dark current become fixed pattern noises, thereby reducing the uniformity of the signal charges. A dark current component due to crystal defects at the surface of semiconductor can be considered as the cause of the dark current. This current arises from charge carriers produced by dopant impurities that recombine at surface defects. In order to reduce that phenomenon, usually a shallow P.sup.+ layer 2 is disposed at the surface of N type layer 1 having a dopant impurity concentration of N.sub.1, and the photodiode 10 is thus buried in silicon. However, in an image sensor which, having such photodiode section 10 and storage section 20, employs charge skimming, even if the dark current of the photodiode section 10 is reduced, the dark current component generated at the storage section 20 opposite the barrier gate 4 and the storage gate 5 will be superimposed on the signal charges, thereby reducing the imaging characteristics.
In the prior art solid state imaging element employing charge skimming, it is quite difficult to reduce the dark current generating at the storage section 20. Reduction of the dark current is limited by the structure of the storage section 20.